The injection molding of foamed plastic articles is regularly practiced by the injection of a molten mixture of plastic and a blowing agent into a mold cavity having a mold surface member movable to enlarge the cavity after the material is injected to allow foaming and expansion of the injected material. Various procedures for such foam molding are disclosed in the U.S. Patent to Beyer et al. No. 3,058,161 of Oct. 16, 1962 and the U.S. Patent to Kyritsis et al. No. 3,801,686 of Apr. 2, 1974. In the procedure of the Beyer et al. patent, the foamable mixture is injected into an expandable mold and resin material adjacent the walls of the mold is solidified to form a thick nonfoamed layer before expanding the mold for foaming of interior portions of the resin in the mold.
The procedure of the Kyritsis et al. patent forms a mixture of resin and chemical blowing agent at a temperature below the foaming temperature and injects the mixture at a high rate through a flow resisting passage rapidly to heat it by viscous dissipation to foaming temperature and fill the mold with unfoamed resin material containing blowing agent during the induction period before development of bubbles from gas from the blowing agent. In this procedure, chemical blowing agent in the resin adjacent the mold walls is arrested by the cooling action of the mold walls to form a thin unfoamed skin and the mold is expanded directly on filling of the mold.
In such methods, it has been proposed to avoid the difficulty that unsightly surface areas develop at the expansion line where the surface members are moved to expand the mold, by use of a special form of mold shown in the U.S. patent to Kyritsis et al. No. 3,596,318 of Aug. 3, 1971. In that mold the surface member movable for expansion projects into the mold cavity at the time of injection and, to expand the mold cavity, is moved out so that its marginal portions are in register with marginal portions of another mold section leaving a mold a mold crease line in the plastic surface at the joint between the surface member and the other mold section.
It has also been proposed in British Patent to Ferrari No. 1,169,394 published Nov. 4, 1969, to resist premature foaming of a molten mixture of plastic and a blowing agent by pressurizing a sealed mold with air before injection of the mixture into the mold. When the mixture is injected, an overpressure valve prevents build up of high uncontrolled pressure, while the retained pressure is stated to ensure smooth glossy surfaces on the molded article. It was found, however, that simple pressurizing of the special form of mold above noted gave rise to unsatisfactory surfaces and mold crease lines.